The invention relates to a method and apparatus for ultrafine grinding of solid particulate material to mean particle sizes far below 1 micrometer resp. to so-called nanosizes and/or for mixing of powder and agglomerate material with mean particle sizes in the range of nanometers (so-called nanopowders) in which the material to be ground and/or mixed (processed) and a grinding/mixing additive are filled into a grinding chamber containing loose grinding media and by motion of the grinding media relative to adjacent media and to the walls of the grinding chamber, the material is ground to the desired particle size and/or is mixed whereupon the additive is separated from the material.
Mills with loose grinding media are employed for ultrafine grinding and mixing of solid materials since long. Such mills are, besides ball mills, vibration mills and agitator mills, also planetary mills. With decreasing size of solid particles, their strength increases. The smaller the particles, the higher the strength of the primary particles or with nanoparticles the strength of ever present agglomerates of particles and the higher the mechanical energy per unit volume required for grinding the primary and agglomerate particles. A lower limit of particle size depending on the material has been observed, below which no brittle fracture occurs. Very fine particles exhibit properties of plastic material. With known methods, nanopowders are only coarsely but not evenly or even completely mixable.
Plastic behavior of the material in combination with high mechanical energy per unit volume transferred to the particles upon the collision of loose grinding media result in compression of fine (ground) particles or fine agglomerates to new strong agglomerates, i.e. re-agglomeration occurs. The high temperatures occurring thereby may even result in sintering with the consequence that agglomerates may exhibit the same strength as the original particles or agglomerates. Therefore a lower limit of achievable particle size exists, which cannot be surpassed with known grinding techniques. This limit depends on the material and is in the range of 1 micrometer.
To reduce plastic behavior, the grinding chamber e. g. of ball mills, vibration mills or agitator mills has been cooled from outside (via a cooling jacket) or from inside (e. g. via the cooled impeller shaft or other inner parts), mostly to temperatures slightly above the freezing point (German utility model 92 08 275), or liquefied gas has been added into the grinding chamber.
For comminution of rubber scrap, liquid nitrogen is sprayed or vaporized in a grinding chamber of a vibration mill (rod mill) which is cooled from outside (U.S. Pat. No. 5,513,809).
For the production of aqueous pigment-dispersions, a filter cake containing 70 to 80% water, for dispersion by comminution, has been frozen partially, i.e. to about 50%, after adding a stabilizer. By agitating with an impeller, e.g. a blade mixer, the agglomerates have been broken up to primary particles of 0.2 to 0.3 micrometer in size and below by means of the formed ice crystals (U.S. Pat. No. 4,013,232).
It has also been tried to comminute particles to sizes of 1 to 20 micrometers (German patent application print 37 02 484) by infiltrating or soaking pre-ground particles of about 50 micrometer size with a swelling fluid, particularly water (supported by ultrasonic waves, if necessary) and then by repeated freezing and thawing. This process is suitable for few materials if at all and is extremely energy consuming.
Prevention of re-agglomeration has been tried by adding additives to the material to be ground and/or mixed. To this end, soft substances, so-called additives, have been added to the material to be processed, e.g. sodium chloride (German laid open patent application print 35 05 024) or graphite, which are softer or more viscoplastic than said material and in which the fragments are held in a dispersed state during grinding. Particles far below 1 micrometer, i.e. nanoparticles, can be produced thereby. The soft additive is removed after grindingxe2x80x94sodium chloride by dissolving in water, graphite by burning off and other additives by dissolving in a solvent.
This method has restrictions and disadvantages. The material completely comminuted or ground and/or mixed, respectively, has to be insoluble in the substance with which the additive is removed after grinding/mixing. In general, contamination by residues of the additive remain, which is not acceptable for many products. If graphite is used as additive and burned off afterwards, then the risk of chemical reactions with the processed material exists.
There is an increasing interest in highly dispersed systems of particles in the range of nanometers. Therefore a grinding and mixing technique becomes necessary which is suitable for grinding and/or mixing of new materials in the field of ceramic materials, materials for the optic and electronic industry, superconducting ceramic materials and compound materials as well as pharmaceuticals.
It is therefore an object of the invention to provide a method and an apparatus being capable of producing particles in the range of nanometers and/or mix them homogeneously, which are not subjected to the restrictions mentioned above and which provide for new applications for materials which heretofore could not be ground to particle sizes far below 1 micrometer and/or which could not be mixed in the range of nanometer sizes.
In order to meet this object a method for ultrafine grinding of solid particulate material to particle sizes far below 1 micrometer and/or for mixing powder and agglomerate material of nanometer sized particles, respectively, is devised which comprises according to the invention the steps of providing a grinding chamber containing loose grinding media, generating a cooled atmosphere in said grinding chamber, feeding said material to be processed and an additive into said grinding chamber, said additive is solidified and below its melting or sublimation temperature and behaves chemically inert with regard to the material to be processed and which can evaporate and/or is volatile at ambient pressure at temperatures below 50xc2x0 C., inducing motion of said grinding media relative to adjacent grinding media and to the walls of the grinding chamber until said material is ground to the desired particle size and/or mixed to the desired mixing state, and thereafter removing said additive from the processed material by evaporation.
The invention further provides an apparatus for ultrafine grinding mixing of solid particulate material to particle sizes far below 1 micrometer and/or for mixing of powder and agglomerate material with particle sizes in the range of nanometers, which apparatus comprises a grinding chamber adapted to be charged with loose grinding media and said material to be processed, means for generating a cooled atmosphere in said grinding chamber, means for feeding said material to be processed into said grinding chamber, means for feeding an additive into said grinding chamber, which additive is solidified and below its melting or sublimation temperature and behaves chemically inert with regard to the material to be processed and which can evaporate and/or is volatile at ambient pressure at temperatures below 50xc2x0 C., said grinding chamber is encased by a cooling jacket with an inlet means and an outlet means for a cooling agent, means for inducing motion of said grinding media relative to adjacent grinding media and to the walls of the grinding chamber until said material is ground to the desired particle size and/or mixed to the desired mixing state, and means for evaporating said additive from said material after grinding and/or mixing.
The feed material and a grinding/mixing additive are fed into a grinding chamber containing loose grinding media and, if applicable, grinding tools (agitator mills) and providing a cooled atmosphere. These material are ground and/or mixed (processed) to the desired particle size and/or mixed by relative motion of the media relative to adjacent media and to the walls of the grinding chamber, and in which thereafter said additive is removed from the processed material. According to the invention the grinding and/or mixing is carried out in a cooled atmosphere in the presence of a solidified additive below its melting or sublimation temperature, which additive behaves chemically inert to the material and which can evaporate and/or is volatile at ambient pressure at temperatures below 50xc2x0 C. Said additive is subsequently removed by evaporation from the processed material.
Thus, the additive has to be in a liquid or vaporous state at ambient or room temperature and the additive shall be a liquid or vapor or gas and shall be in a solid aggregate state during grinding/mixing. Well suitable additives are water ice or carbon dioxide ice (solid carbon dioxide) or comparable substances like refrigerating agent R134a. Temperatures below about xe2x88x9230xc2x0 C., especially below xe2x88x9250xc2x0 C. are for grinding/mixing with water ice, below xe2x88x9280xc2x0 C. for grinding/mixing with carbon dioxide ice.
For cooling the atmosphere in the grinding chamber down to a low temperatures which prevents melting or evaporation of the additive, appropriately cooled cooling agents are useful but also liquefied gases like liquid nitrogen.
The addition of fine-grained water ice or solid carbon dioxide and the grinding/mixing at low temperatures have the advantages of a careful treatment of the material and that no contaminations are left in the product. A re-agglomeration of ground/mixed very fine particles is suppressed.
Grinding apparatus of known type as said vibration mills and agitator mills can be used after making modifications necessary for coping with and for the cooling to very low temperatures. A vibration mill with water as coolant is known, which can be operated at temperatures not much below 0xc2x0 C. For that purpose a cooling jacket with in- and outlets for the cooling water is provided encasing the grinding chamber. In the present invention, however, a cooling jacket and a grinding chamber have to be provided, which resist the very low temperatures of a cooling agent even during grinding/mixing operations. The cooling agent is being cooled to the necessary very low temperatures by a refrigerating machine, if not supplied in a liquid state. The cooling capacity has to be so large that the electric power of the millxe2x80x94which will be nearly completely converted into heatxe2x80x94can be transported away. For agitator or vibration mills usually a jacket encasing the grinding chamber is sufficient since grinding media and material being ground are sufficiently circulated and transported continuously to the walls of the grinding chamber where the heat is removed. Agitator mills require additional cooling of the impeller shaft to guarantee intense heat exchange.
A discontinuously running vibration mill is operated with the following steps:
1. Cooling of the grinding chamber by filling liquid nitrogen into the jacket encasing the grinding chamber;
2. charging of the cooled mill via an opening with grinding media, material to be ground of appropriate particle size, pre-cooled, if necessary, particularly with a mean particle size below about 20 micrometers, or nanopowders to be mixed, respectively, and a cooled solid, fine grained particulate additive;
3. operation of the mill, grinding or mixing, respectively, of the particles;
4. shutting off and heating up of the mill, discharging the ground/mixed material;
5. evaporating or thawing the additive; and
6. in case of water ice as additivexe2x80x94drying the ground/mixed material.
In this case, the mill is operated discontinuously (batchwise). Continuous grinding is also possible, if appropriate flexible and thermally insulated inlet and outlet ducts are used. In addition, feed material has to be cooled and fine-grained additive to be generated and fed continuously to the grinding chamber while operating it. Furthermore, the ground or mixed material has to be discharged continuously, if necessary to be separated from discarded grinding media which have to be recirculated into the grinding chamber in a closed circuit.
Fields of use of the invention are production of nanoparticles of pharmaceutical substances, especially using solid carbon dioxide as an additive, rarely water ice. By grinding at low temperatures even sensitive substances will not be damaged. A standard grinding at low temperatures without the addition of additives would not result in nanoparticles.
The invention can also be used for the production of high-purity nanoparticles for nanostructured materials (ceramics, metals, nano-compound materials, opto-electronic nano-materials). Finally, the invention is suitable for the mixing of nanopowders which were produced by other methods. In general, it is extremely difficult to mix nanoparticles homogenously.